The present invention relates to a novel and inexpensive way of providing heat transfer plates with a layer of a surface protecting material, which method is characterized in that the heat transfer plates are first assembled to form a plate heat exchanger, in which plate interspaces for the through flow of two heat exchange fluids are formed, and that a gaseous medium containing the surface protecting material is then introduced into the plate heat exchanger at least in those plate interspaces being intended for through flow of one of said heat exchange fluids, the surface protecting material, in a way known per se, being caused to form a layer on the surfaces of the heat transfer plates.
The method according to the invention may be applied to any kind of plate heat exchanger but is particularly intended for plate heat exchangers having permanently assembled heat transfer plates, e.g. welded or brazed plate heat exchangers. In brazed plate heat exchangers the invention provides in addition to a surface protection for the heat transfer plates also a surface protection for the brazing material used between the heat transfer plates. This means that the field in which brazed plate heat exchangers may be used can be extended, since the brazing material sometimes constitutes an obstacle to using such heat exchangers in connection with certain liquids.
It is particularly suitable in connection with use of the invention that a plastic be used as a surface protecting material. The plastic material may be introduced into the plate heat exchanger either in the form of a mist, i.e. in the form of small liquid drops suspended in a gas, or in an evaporated form. In the latter case the gaseous medium being introduced into the plate heat exchanger may completely consist of evaporated plastic. A technique that may be used is described in the World Patents Index abstract of Patent SU 1151546, which reads as follows:
Poly-p-xylylene films and coatings are used in electrical equipment for insulation. They are made by thermal decomp. of di-p-xylylene at 450-700 deg. C., 1-100 mm Hg press. The pyrolysis products are condensed and polymerized as a layer at 15-25 deg. C. The electrical strength of coatings and films is increased by maintaining the initial layer in air, after which the condensation and polymerization processes are repeated 2-5 times. PA1 Typically, 1 g-di-p-xylylene is placed in a sublimation chamber, the pressure is lowered to 1 mm Hg and the chamber is heated to 200 deg. C. The pyrolysis zone consists of a quartz tube, 600 mm long, dia. 18 mm, maintained at 600 deg. C. The di-p-xylylene molecule is decomposed into 2 molecules of p-xylylene, which are carried in a gas stream to the polymerization zone maintained at 25 deg. C. The process is repeated 4 times, so that 5 bonded layers are produced, thickness 20 microns, electrical strength 500 kV/mm, resistance under rupture 62 MPa, rel. elongation 21 percent. PA1 ADVANTAGE--The patented process raises the electrical strength of these films from 220 to 530 kV/mm, without deterioration of other characteristics. Bul. 15/23.4.85@ (2pp Dwg. No 0/0).
Thus, according to the technique described in the foregoing abstract, a substance, di-p-xylylene, may be transformed from a solid state to a gas by sublimation in a first chamber at a pressure of 1 mm Hg and a temperature of 200.degree. C., after which the substance is subjected to pyrolysis in a second chamber at 600.degree. C. A substance, e.g. di-p-xylylene, treated in this manner would then according to the invention be sucked into an assembled plate heat exchanger kept at a relatively low temperature. e.g. room temperature, so that the gaseous substance is condensed onto the surfaces of the heat transfer plates in the plate heat exchanger. Simultaneously, a polymerization takes place. Sucking of the gaseous plastic material into the plate heat exchanger may be performed in several steps to obtain several layers of plastic on the surfaces in question. If necessary for a sufficient covering of all the surfaces by layers of plastic, the suction of the gaseous plastic material into the plate heat exchanger may be performed alternately from the inlet and the outlet, respectively, of the plate heat exchanger for the heat exchange fluids.